Body mass index (BMI), which is weight relative to height in kg/m2, is used to categorise adults as underweight, normal weight, overweight or obese. However, BMI does not account for the amount and distribution of fat and muscle.

Central obesity is a store of excess fat around the middle of the body and is a marker of abnormal fat distribution. This study tested the hypothesis that people with normal weight and central obesity would have more heart problems than people with normal weight and normal fat distribution.

In 1997 to 2000 the study enrolled 1,692 residents of Olmsted County, Minnesota, aged 45 years or older. The sample was representative of the county population for age and sex. Participants underwent a clinical examination and measurements were taken of weight, height, waist circumference and hip circumference. Central obesity was defined as a ratio dividing the waist circumference by the hip circumference of 0.90 or above for men and 0.85 or above for women.

Patients were followed-up from 2000 to 2016 for the occurrence of major adverse cardiovascular events (MACE) using linked medical records from the Rochester Epidemiology Project. MACE was defined as heart attack, surgical or percutaneous coronary revascularisation to open blocked arteries, stroke, or death from cardiovascular causes.

Participants with a normal BMI (18.5-24.9 kg/m2) and central obesity had an approximately two-fold higher long-term risk of MACE compared to participants without central obesity, regardless of their BMI.

Medina-Inojosa said: "People with a normal weight but a fat belly have more chance of heart problems than people without a fat belly, even if they are obese according to BMI. This body shape indicates a sedentary lifestyle, low muscle mass, and eating too many refined carbohydrates."

"The belly is usually the first place we deposit fat, so people classified as overweight BMI but without a fat belly probably have more muscle which is good for health," he continued. "Muscle is like a metabolic storehouse and helps decrease lipid and sugar levels in the blood."

Participants with a normal BMI and central obesity also had a higher risk of MACE than overweight and obese participants with central obesity. Medina-Inojosa said that overweight and obese people with central obesity might also have more muscle mass which could be protective.

The study was presented at EuroPrevent 2018, a European Society of Cardiology congress.